We have previously characterized mouse and human cDNA clones encoding a new transmembrane EGF-like protein. The predicted protein from this cDNA (termed delta-like or dlk) is highly homologous to invertebrate homeotic transmembrane proteins, including Notch and Delta of Drosophila. These proteins also contain EGF-like repeats at their extracellular domains which intervene in receptor-ligand interactions between cells. These interactions generate signals which are involved in differentiation decisions. The function of dlk is unknown. Studies in our laboratory have revealed that Pref-1, as a new EGF-like molecule able to inhibit the adipocyte differentiation of 3T3L1 cells. is a polymorphic variant of dlk. Other polymorphisms, as well as alternately spliced dlk species, have been identified. The 3T3L1 model is currently being used to study the mechanism by which dlk blocks 3T3L1 adipocyte differentiation. We have found that down regulation of dlk expression is necessary, but not sufficient, to inhibit 3T3L1 differentiation. The homology with Drosophila Delta suggests that dlk could act as a ligand for a member of the mammalian Notch family of proteins, which were identified by their putative role in human tumorigenesis, including mammary carcinoma and leukemogenesis. Crosstalk interactions between dlk and the Notch family are being studied in the 3T3L1 cells model. We have observed that 3T3L1 cells with diminished expression of Notch proteins are unable to differentiate to adipocytes. 3T3 L1 differentiation can also be blocked by addition of a recombinant protein encompassing the domains known to intervene in Notch1 interaction with its ligands. Future investigations will focus on the role of dlk and Notch proteins on the modulation of the signal transduction pathways involved in adipocyte differentiation.